I'm a longtime reader and International Affairs student at GWU, less than 2 miles from where the protest today took place. Though I'm certainly no friend of the quite openly socialist anti-war movement, I have to say that I find that report that you linked to quite deceiving. There were certainly far more people there than the linked pundit suggests- my guess is 40-50,000 people. This number is way below the turnout the organizer were expecting, but the crowd was not dainty.
Seeing as I don't usually frequent rallies of this sort, one thing that struck me as particularly odd was how rough the overall message was. Dozens of different groups each with their own agendas were handing out posters, newsletters, and in one case, I was handed a so-called "hit sheet" for the top "War Profiteers" in Iraq, loaded with the usual suspects: Lockheed, Boeing, etc. The movement is loosely organized at best, and hardly the force it thinks itself to be. Simply walking around for a couple of hours led me to a couple of observations:
1) The majority of the people marching/at the rally probably also marched on Washington in the 60's, or are trying to make up for the fact that they missed out on the spectacle the first time. The number of college aged protesters was surprisingly low, and most of the ones that did show up were directly attached to one socialist campus group or another. Possibly as a result of this, the energy level of the protest wasn't exactly stunning, and the police looked miserably bored for the most part.
2) It didn't take long to figure out that most of the planning and promotion for the protest had gone into getting people there. There was low level confusion as to what was going on throughout most of the day, and the noticeable lack of focus caused more than a few protesters to wander off. For example
3) I saw three groups of counter protesters. One in front of the National Art Gallery, one just north of the Capitol, and a small group of young Marines standing with a life-size cutout of President Bush. The Marines were taking quite a beating from the crowd, and were definitely keeping their cool quite well. The entire counter-protest probably numbered less than 45-50 in total, and the slogans being used and chanted were just as tired as those on the liberal side of the fence.

Protests are tired in general. Media reporting, however, seems selective -- there wasn't much reporting on the "pro life" protests in Washington on Monday, perhaps because there's no way to spin that against Bush.
ANOTHER UPDATE: Reader Christian Aranda emails:

This has been pointed out elsewhere, but I thought I'd let you know that as a DC resident walking around picking up groceries, and stopping by the hardware store, it was obvious that most of the people holding the signs at the anti-war rally in DC were over 50.
This sounds awful, but I saw some really ancient looking people walking around with signs. The first thing that came to mind was whether or not they had children who fought in Vietnam. They were old, Glenn, and I don't mean in a 50 is the new 40 way. I'm talking nursing home old.
Thought you might find this interesting.